Issue clearing previous selection

I am having a small problem, that is giving me a headache

I have a form with 2 comboboxes on it, the option appearing in the second being dependent on the selection from box 1.

during the initial pass then no problems, and box two only shows the correct items, but if the user realises that they have chosen the wrong selection in box 1, then when displaying the avaialble options for box 2, it is showing the originals options (ie from the first selection) as well as the options for the new selection.

How to clear the options from cache or memory when the user goes back to reselect an option from box 1

As an example

Box 1 has 2 selections being 1,2. Correct values for box two based on box 1 would be

Box1 = 1 a,b,c,d
Box2 = e,f,g,h

when on the second pass then box2 would show a,b,c,d,e,f,g,h


Thanks to all in advance

Timer,

May be I am obtuse. Your problem wasn't very well sold to me and came across as a bit addled. Could you kindly post the code you have thus far, so what you are describing could be gleaned from it.

Many thanks Timer. Yor 2nd rendition of the problem is as lucid as it can get.
I am glad the issue is resolved with CLEARING on exit. Sounds the right thing to do.